1. Field of the Invention
The invention relates to wireless communications technologies, and in particular to a system and method for providing an efficient GSM-EDGE dual-mode transmitter.
2. Related Art
Communications using the global system for mobile telecommunication (GSM, see 3GPP(™) Technical Specification 45.005 v5.5.0 (August 2002)) system are transmitted within two predetermined transmission spectra—890–915 MHz in the uplink direction (i.e., from the mobile station to the base station) and 935–960 MHz in the downlink direction (i.e., from the base station to the mobile station). To make efficient use of these transmission spectra, bandwidth allocation to individual users is provided via frequency division multiple access (FDMA) and time division multiple access (TDMA).
FDMA is used to divide the transmission spectra into multiple carrier frequencies of 200 kHz. Each carrier frequency is then divided into 4.615 ms frames, and TDMA is used to divide each frame into eight time slots, or burst periods. Each of the eight burst periods is assigned to a single user, and a communications signal from that user must be modulated into discrete “burst output signals” that are transmitted only within that user's burst periods. In this manner, a single carrier frequency can handle up to eight simultaneous users.
To ensure that signal quality and integrity is maintained in the shared-channel approach used by GSM, it is important that burst output signals are precisely constrained within their associated burst periods. If a burst output signal turns on (i.e., ramps up) or turns off (i.e., ramps down) too quickly, the RF signal can spill out of the allowable transmission spectrum (“signal splatter”) and interfere with the transmissions of other GSM users. However, if the burst output signal turns on or turns off too slowly, the user's own data transmission may be corrupted. Therefore, the GSM specification carefully defines a timing mask that provides an allowable power-time template (“burst profile”) for a GSM burst output signal.
FIG. 1A shows a sample timing mask T(1) for GSM communications, which consists of an upper boundary U(1) and a lower boundary L(1). A GSM-compliant burst output signal (such as sample burst output signal S(B1)) must be confined between upper boundary U(1) and lower boundary L(1). Those boundaries define a burst profile that can be divided into three segments—a “ramp up” segment between times T0 and T1, a “data burst” segment between times T1 and T2, and a “ramp down” segment between times T2 and T3. So long as the ramp up and ramp down portions of burst output signal S(B3) fall between upper boundary U(1) and lower boundary L(1), problematic signal splatter and data corruption can be avoided.
The data burst is the portion of the burst output signal in which the actual communications data is transmitted. GSM uses Gaussian Minimum Shift Keying (GMSK) to modulate the communications data onto the selected carrier frequency. GMSK uses a two-state modulation algorithm that was originally selected to provide good power efficiency and acceptable bandwidth for mobile phone users. Because GMSK is a minimum shift keying modulation scheme, the data burst provided by a conventional GSM transmitter has a constant envelope (i.e., the amplitude of the signal does not change).
FIG. 1B shows a schematic diagram of a conventional GSM transmitter 100 that could be used to provide GSM burst output signal S(B1) shown in FIG. 1A. GSM transmitter 100 includes a baseband chip 102, a GMSK modulator 110, a local oscillator 120, and a power amplifier 130. Baseband chip 102 includes a data conversion digital-to-analog converter (DAC) 101A, a ramp control DAC 101B, and a smoothing filter 103. DAC 101 converts GSM digital burst data (i.e., the bits used to generate the burst output signal for a GSM burst period) into an analog baseband signal S(D1) (smoothed by smoothing filter 103) that GMSK modulator 110 then modulates onto an appropriate carrier frequency supplied by local oscillator 120. Typically, analog baseband signal S(D1) includes both an in-phase (“I”) component and a quadrature (“Q”) component that are modulated into a modulated signal S(M1) by GMSK modulator 120. Modulated signal S(M1) is then amplified by PA 130 into burst output signal S(B1).
As described above with respect to FIG. 1A, GMSK modulation is a constant envelope modulation technique. Therefore, PA 130 can be operated in saturated mode (i.e., as a voltage-limited amplifier) without introducing signal-degrading distortion into burst output signal S(B1). If PA 130 is operated in saturated mode, the amplitude of burst output signal S(B1) can be adjusted by changing the saturated (fully-on) output of PA 130. Therefore, to generate a GSM-compliant burst output signal S(B1) (i.e., a baseband signal S(D1) that has a power-time profile that fits the timing mask shown in FIG. 1A), the saturated output level of PA 130 must always fall within the limits defined by GSM timing mask T(1) shown in FIG. 1A.
Ramp control DAC 101B generates a ramp control signal S(R1) that regulates the gain provided by PA 130. Ramp control signal S(R1) is typically a feedback loop that compares the output level of PA 130 against the desired ramp profile (as shown in FIG. 1A). During the ramp up and ramp down portions of burst output signal S(B1), ramp control DAC 101B increases or decreases, respectively, the gain provided by PA 130 so that burst output signal S(B1) exhibits a GSM-compliant profile.
Note that the use of saturated amplification allows GSM transmitter 100 to be simply implemented while maintaining good power efficiency. Due in part to these factors, GSM is presently the most widely used cellular phone technology. However, the relatively low data rate of GSM is a limiting factor as mobile communications move beyond simple voice transmissions into more bandwidth-intensive transmissions (e.g., images and video). Therefore, the Enhanced Data for Global Evolution (EDGE) system has been developed as a new high-speed mobile data standard (3GPP Technical Specification 45.005).
EDGE is designed to provide a higher data rate (in equivalent occupied bandwidth) compared to conventional GSM technologies, but at the same time retain compatibility with the existing GSM infrastructure for maximum utility. EDGE uses eight-phase-shift-keying (8PSK) modulation that makes use of eight modulation states using three modulation bits, in contrast to the single modulation bit used by GMSK. However, the improved data efficiency of EDGE comes with added modulation complexity. Specifically, because EDGE uses 8PSK modulation, both the phase and the amplitude of the signal are modulated, and the data burst of an EDGE transmission is no longer a constant envelope signal.
FIG. 2A shows a sample timing mask T(2) for EDGE communications, which consists of an upper boundary U(2) and a lower boundary L(2). An EDGE-compliant burst output signal (such as sample burst output signal S(B2)) must be confined between upper boundary U(2) and lower boundary L(2). Like GSM burst output signal S(B1) shown in FIG. 1A, EDGE burst output signal S(B2) can be divided into three segments—a “ramp up” segment between times T0 and T1, a “data burst” segment between times T1 and T2, and a “ramp down” segment between times T2 and T3.
To maintain compatibility with current GSM systems, the ramp up and ramp down portions of EDGE burst output signal S(B2) are constrained in much the same manner as GSM burst output signal S(B1) shown in FIG. 1A. However, because of the 8PSK modulation used by EDGE, the data burst portion of EDGE burst output signal S(B2) is a non-constant envelope signal that can exhibit substantial amplitude variation.
Because of this data burst amplitude variation, an amplifier for an EDGE transmitter must be able to operate in the linear mode to avoid clipping and other forms of signal distortion. However, the amplifier should also be able to operate in the saturated mode to achieve good power efficiency when operating in GSM mode. Therefore, a GSM/EDGE (dual mode) system requires a dual-mode amplifier (i.e., an amplifier that operates in both the linear and saturated regions). Conventional EDGE amplifiers provide this dual-mode capability by adjusting the base biasing of the power transistors providing the amplification within those EDGE amplifiers.
For example, FIG. 2B shows a schematic diagram of a conventional EDGE transmitter 200 that could be used to provide EDGE burst output signal S(B2) shown in FIG. 2A. EDGE transmitter 200 includes a baseband chip 202, a GMSK/8PSK modulator 210, a local oscillator 220, a variable gain amplifier (VGA) 230, and a power amplifier (PA) 240. Baseband chip 202 includes a data conversion DAC 201A, a ramp control DAC 201B, and a smoothing filter 203, and also generates control signals VGA_CTRL and MODE.
During operation of EDGE transmitter 200, digital burst data (the bits for either a GSM burst output signal or an EDGE burst output signal) is converted by DAC 201A into an analog baseband signal S(D2) (smoothed by smoothing filter 203). GMSK/8PSK modulator 210 modulates baseband signal S(D2) onto an appropriate carrier frequency supplied by local oscillator 220. The resulting modulated signal S(M2) generated by GMSK/8PSK modulator 220 is then amplified by VGA 230 and/or PA 240 into burst output signal S(B2).
The type of modulation applied by GMSK/8PSK modulator 210 and the manner of amplification provided by VGA 230 and PA 240 is controlled by control signals MODE and VGA_CTRL. Control signal MODE is placed in either a GSM state or an EDGE state, depending on whether transmitter 200 is operating in GSM or EDGE mode, respectively. Baseband chip 202 provides control signal MODE to both GMSK/8PSK modulator 210 and PA 240, while providing control signal VGA_CTRL to VGA 230.
If transmitter 200 is operating in GSM mode, baseband chip 202 places signal MODE in a GSM state, which causes GMSK/8PSK modulator 210 to apply GMSK modulation to signal S(D2). Baseband chip also generates a control signal VGA_CTRL that causes VGA 230 to provide a fixed, high gain. Meanwhile, in response to control signal MODE being in the GSM state, PA 240 applies saturated mode amplification to modulated signal S(M2). This saturated mode amplification is controlled by a ramp control signal S(R2) provided by ramp control DAC 201B, in a manner substantially similar to that described with respect to FIG. 1B.
However, if transmitter 200 is operating in EDGE mode, baseband chip 202 places signal MODE in an EDGE state, which causes GMSK/8PSK modulator 210 to apply 8PSK modulation to signal S(D2). Placing signal MODE in the EDGE state also causes PA 240 to provide a linear gain, so that amplification of modulated signal S(M2) is provided by both VGA 230 and PA 240.
The linear mode amplification provided by VGA 230 is required because of the non-constant envelope of 8PSK-modulated signal S(M2). For example, when generating the data burst portion of burst output signal S(B2), the data-carrying amplitude variations of modulated signal S(M2) must be preserved. Therefore, a linear amplifier (i.e., VGA 230) must be used to provide the necessary signal gain, since a saturated amplifier (i.e., PA 240) will destroy any envelope variations.
The portions of modulated signal S(M2) that are amplified into the ramp up and ramp down portions of burst output signal S(B2) do not carry any transmission data. However, those portions of modulated signal S(M2) still exhibit non-constant envelopes due to the nature of analog signal S(D2) provided to modulator 211. Therefore, linear amplification must also be employed to prevent spectral splatter during the rapid ramp up and ramp down portions of burst output signal S(B2). Applying only saturated mode amplification in this situation (high gain applied to a non-constant envelope signal) can generate unwanted spectral components (spectral splatter) in burst output signal S(B2), thereby resulting in a non-compliant EDGE signal (i.e., a signal that does not meet the EDGE spectral mask specifications shown in FIG. 2C).
Unfortunately, providing the degree of control over VGA 230 required to generate an EDGE compliant burst output signal S(B2) can be very difficult due to the indirect nature of the control. Typically, the gain provided by a VGA is controlled by regulating the biasing of the amplifying transistor(s) in the VGA. VGA control provides a straightforward means for controlling the range of inputs to which linear amplification can be applied. However, providing the precise output control required to maintain compliance with the EDGE timing mask can be difficult to provide using only VGA control, since the magnitude of the output signal (i.e., burst output signal S(B2)) is dependant on the level of modulated signal S(M2), the gain of VGA 230, and the gain of PA 240. In addition, PA 240 is subject to gain expansion, which makes precise gain control even more difficult.
Further complicating the effort is the fact that because linear amplification is required for EDGE transmissions, VGA 230 must have a wide dynamic range. For example, the basic ramp up for GSM or EDGE requires 35 dB, while the particular transmit power control level (PCL) used for the transmission can impose another 23 dB offset. Therefore, to ensure that VGA 230 can operate in the linear mode for EDGE transmissions, VGA 230 must have a dynamic range of at least 58 dB (35 dB+23 dB). Having such a wide dynamic range would typically require a significant number of amplifier stages, which creates greater opportunity for accuracy degradation through process variations (during manufacturing) and temperature variations (during operation). A VGA that provides accurate linear response across such a wide dynamic range can be difficult to realize cost effectively.
Accordingly, it is desirable to provide an EDGE transmitter that does not require a high-precision, high-dynamic range amplifier.